1. Field of the Invention
The present invention relates to a positive electrode for a non-aqueous electrolytic secondary cell and a non-aqueous electrolytic secondary cell which have a high potential, a high capacity, safety, and an excellent cycle characteristic.
2. Description of the Related Art
In recent years, since the non-aqueous electrolytic secondary cell represented by a lithium secondary cell is smallest, lightest, chargeable and dischargeable with a high capacity, the cell is put to practical use as a power supply of a cellular phone demanded to be small-sized and light-weight, a portable electronic device such as a personal computer or a video camera, or a communication device. The cell is expected as a cell for driving a motor of an electric vehicle (EV) or a hybrid electric vehicle (HEV), or as means for storing power at night for effective use of electricity.
Typical examples of a positive electrode material of the non-aqueous electrolytic secondary cell mainly include lithium transition metal compound oxides such as lithium cobalt oxide (LiCoO2), lithium nickel oxide (LiNiO2), and lithium manganate (LiMn2O4).
Here, a high charge and discharge capacity is obtained in the cell using lithium cobalt oxide or lithium nickel oxide. However, since thermal stability of lithium cobalt oxide or lithium nickel oxide itself is low, safety of the cell is not sufficient when the cell is exposed to the abnormal conditions, for example, in the case that heat is generated abnormally. Especially, the cell wherein lithium nickel oxide is used as a positive electrode does not function well when the amount of desorbed Li from lithium nickel oxide increases. In addition, there is a problem that released oxygen from lithium nickel oxide is apt to react with an electrolysis solution, and in the worst case the cell will be ruptured. Moreover, lithium cobalt oxide has a problem in that the cell becomes very expensive in the case where a large-sized cell is prepared, because a producing area of cobalt as a raw material is limited, and the resource amount thereof is little. There is also a problem in that the output density of lithium cobalt oxide is small compared with that of lithium manganate.
On the other hand, in case of the cell using lithium manganate, a large-sized cell can be inexpensively prepared since there is mainly used as a positive electrode material spinel type lithium manganate whose resource amount is ample, which is inexpensive and which is excellent in safety. Thus, the safety even under the conditions that the disorder is caused in the cell can be improved compared with lithium cobalt oxide and lithium nickel oxide.
However, since the cell using lithium manganate has a smaller charge and discharge capacity as compared with the cell using lithium cobalt oxide or lithium nickel oxide, there are problems in that sufficient cell characteristics cannot be obtained and in that the capacity at high temperature decreases remarkably. As one of causes for the drop in capacity, the following phenomenon is supposedly generated in a cell system. That is, in the non-aqueous electrolytic secondary cell prepared using, for example, an LIPF6-based electrolysis solution as an electrolysis solution, HF is generated in the system especially at a high temperature. Accordingly, it is considered that a part of Mn is eluted from lithium manganate, and this phenomenon deteriorates the positive electrode active material and affects adversely on a negative electrode active material. As a result, it is supposed that there is caused a disadvantage such as the above-described capacity drop at high temperature.
To solve the above-described problem, a method of substituting a part of Mn by another metal element and a method of coating the surface of lithium manganate has been investigated. In recent years, there has been proposed a method of mixing lithium cobalt oxide or lithium nickel oxide with lithium manganate to obtain a positive electrode active material of the non-aqueous electrolytic secondary cell (See, for example, JP-A-2003-197180, JP-A-2001-143705, and JP-A-2000-215884). However, under the present circumstances, a sufficient effect has not been necessarily achieved.
Moreover, in recent years, investigations have been made with respect to using a phosphate compound having an olivine structure, represented by LiFePO4 as the positive electrode active material of the non-aqueous electrolytic secondary cell, and the compound is noted as the next-generation positive electrode active material for the non-aqueous electrolytic secondary cell. Phosphorus (P) and oxygen (O) in the phosphate compound have a very strong covalent bond. Even in a case where the amount of desorbed Li increases as in lithium nickel oxide described above, oxygen (O) is not easily liberated from phosphorus (P). Therefore, oxygen (O) hardly reacts with the electrolysis solution, and the thermal stability is very high. In the phosphate compound, LiFePO4 has a capacity of 3 V whereas the olivine type lithium manganese phosphate (LiMnPO4) has a capacity of 4 V, which corresponds to about 1.5 times capacity of the spinel type lithium manganate per molar ratio of Mn. However, since the phosphate compound itself has low electric conductivity, the compound has a problem in that the inner resistance of the cell increases, and the rate characteristics remarkably decrease in the case where the cell is prepared using the phosphate compound alone as the positive electrode active material.